Error correction of amplifiers grows ever more important as frequency allocation, frequency reuse, and RF interference (RFI) become more prevalent. All amplifiers produce distortion products, or error signals, as a part of the amplification process. The amount of error varies, but is primarily due to the operational class of the amplifier. Class C amplifiers, while very efficient, generate significant error signals. Class AB amplifiers operate somewhat less efficiently, producing less error than a Class C amplifier operating at similar power levels. Class A amplifiers provide the lowest level of error signal, but at a higher cost such that the power efficiency of the amplifier is very poor. As a tradeoff, one can use a Class AB amplifier for a specific application such as wireless telephony transmission, and utilize associated circuitry operating in the feed forward cancellation mode for reducing the error components generated by the amplifier. This provides reduced error levels at a reasonable level of operational efficiency.
The feed forward error control concept was originated in the 1920's by Harold S. Black and described in his U. S. Pat. No. 1,686,792 issued Oct. 9, 1929. The concept is more fully described in an article entitled "A Microwave Feed-Forward Experiment", by H. Seidel, published in The Bell System Technical Journal, Vol. 50, No. 9, November, 1971. Its important properties are that it incorporates time, phase and amplitude compensation to reduce error signals produced by the amplifier. Compensation of these three parameters allows operation at much higher frequencies, and over much greater bandwidths, than other types of error control such as negative feedback. Also, because time compensation is incorporated into the system, the ultimate performance of the system becomes dependent upon the physical component variations, and not upon limitations due to transit time and associated phase shift through the system.
The feed forward amplification process involves signal amplification, recognition and amplification of the undesired signals (errors), and combination of properly compensated error signals with the distorted amplifier output signal (herein, "amplified and distorted message signal") so as to produce a corrected final output signal in which the level of the error signals is reduced by cancellation or destructive interference. Associated circuitry includes an error correction circuit for detecting message signal error in the operation of the main amplifier and producing an amplified message signal error which is then subtracted from the amplified and distorted message signal output of the main amplifier to produce a corrected final output signal.